docs: def is_not_null(df: DataFrame, col_name: str) -> bool:
      """
      Return ``True`` if the given DataFrame has a column of the given
      name (string), and there exists at least one non-NaN value in that
      column; return ``False`` otherwise.
      """
      if (
          isinstance(df, pd.DataFrame)
          and col_name in df.columns
          and df[col_name].notnull().any()
      ):
          return True
      else:
          return False